This is possible, but not easy to answer without seeing the code that calls your overlay.

Basically you would look if your modal function provides a callback (a javascript function that is run after the code has executed) and provide a "virtual pageview", i.e. run a pageview call and pass in a custom path that indicates the modal window:

ga('send', 'pageview', '/modal/path/to/post/');

The path you provided as extra parameter will appear in your content reports as a page. If you also want to provide a title (or override any other field in GA) you can pass a JSON object as third parameter: